1. Field of the Invention
The present invention relates to a power transmission apparatus used for four-wheel drive vehicles, for instance, and more specifically to a power transmission apparatus for transmitting power via viscous resistance of viscous fluid applied to a plurality of resistance plates arranged within a working chamber of the apparatus.
2. Description of the Prior Art
An example of the above-mentioned power transmission apparatus is disclosed as a viscous coupling apparatus in Japanese Published Unexamined (Kokai) Patent Application No. 58-50349. In this viscous coupling apparatus, a plurality of resistance plates are arranged on the casing side and the shaft side, respectively along the axial direction thereof so as to be alternately opposite to each other and extend in the radical direction thereof within a working chamber filled with a viscous fluid (e.g. silicon oil). Therefore, when either of the casing or shaft is rotated, power is transmitted from one to the other via fluid viscous resistance generated by relative rotation between these resistance plates.
In the above-mentioned prior-art power transmission apparatus or viscous coupling apparatus, however, the working chamber is airtightly formed by sealing members disposed between the casing and two accel shafts and further the resistance plates fixed to the shaft side within the working chamber filled with viscous fluid are arranged along splines formed on the outer circumferential surface of an end of each of the two accel shafts. Therefore, it has been impossible to subassemble the apparatus after the viscous fluid is put into the working chamber. In other words, since the viscous fluid is put after the resistance plates have been assemble and further the sealing members have been attached during vehicle assembly process, there exists a problem in that vehicle assembly work is complicated and therefore vehicle assembly reliability is deteriorated.
To overcome the above-mentioned problem, another prior-art viscous coupling apparatus is disclosed in Japanese Published Unexamined (Kokai) Patent Appli. No. 62-165032. In this apparatus, the working chamber is formed by an outer cylindrical member, two side flanges formed integral with the outer cylindrical member on both the sides thereof, and a hub arranged on the inner circumferential side of the side flanges. Further, a plurality of resistance plates are arranged along the axial direction and alternately fixed to the inner circumferential wall of the, outer cylindrical member and the outer circumferential surface of the hub so as to extend in the radical direction within a working chamber filled with viscous fluid. The inner circumference of the hub is linked with an input shaft to which power is applied and further loosely fitted to the side flanges. Sealing members are disposed at the fitting portion between the hub and the side flanges to form an airtight working chamber.
The above-mentioned second prior-art viscous coupling apparatus has such an advantage as to enable the apparatus to be subassembled; however, since a hub is provided along the axial direction, there exists another problem in that the volume or the radical length of the working chamber is reduced and therefore it is difficult to obtain desired performance. To overcome this problem when the axial length of the working chamber is increased, the apparatus is inevitably increased both in size and weight.
In addition, since the inner circumference of the hub must be centered with respect to the apparatus axis and further the outer circumference of the hub must be centered with respect to the outer cylindrical member, there exist other problems in that the manufacturing process and assembly process are difficult.